Switched telecommunication networks are designed in such a way that the individual exchanges are more or less intermeshed. This means, on the one hand, that a connection can be established from any exchange of the network to any other exchange and, on the other hand, that as a rule, there is not only one possibility of routing the call, but more than one possibility, frequently three or more possibilities. This not only affords safety against disturbances but also results in a more uniform load distribution. There are many strategies to select an optimum possibility for each call attempt. In any case, at least the most recent attempts and their results are taken into account.
A problem arises from the fact that not all of the available possibilities are equivalent. On the one hand, individual ones of the possible paths are more expensive than others, and on the other hand, the capacities of the paths are partly very different.